Article pubs.acs.org/IECR
Steam-Enhanced Calcium Looping Cycles with Calcium Aluminate Pellets Doped with Bromides Vasilije Manovic,† Paul S. Fennell,‡ Mohamad J. Al-Jeboori,‡ and Edward J. Anthony*,§ †
CanmetENERGY, Natural Resources Canada, 1 Haanel Drive, Ottawa, Ontario, Canada K1A 1M1 Department of Chemical Engineering, Imperial College, London SW7 2AZ, United Kingdom § School of Applied Sciences, Cranfield University, Cranfield MK43 0AL, United Kingdom ‡
ABSTRACT: This study explores the effect of calcium bromide (CaBr2) doping of lime-based sorbents in the presence of steam during calcination/carbonation cycles. Two sorbents were tested: natural limestone (Cadomin, Canada) and a synthetic sorbent (pellets) prepared from Cadomin limestone with addition of calcium aluminate cement. The mixture of calcined limestone and cement was pelletized in a mechanical granulator that uses spray water as the part of the pelletization process. Both the original limestone and the prepared pellets were impregnated with a dilute CaBr2 solution to achieve a Ca/Br mole ratio of 500:1. The CO2 carrying activities of the sorbents were tested during calcination/carbonation cycles in a thermogravimetric analysis (TGA) apparatus. Realistic calcination conditions during the reaction cycles were employed: 900 °C with a CO2 sweep gas. Multicycle tests were carried out with steam 15% and without steam present in the carbonation gas stream (20% CO2, 15% steam or 0% steam, N2 balance in both cases). The results showed that doping with CaBr2 has a beneficial effect on sorbent CO2 capture activity, and in particular, the conversion rate during the diffusion-controlled stage of carbonation was found to exhibit a strong synergic enhancement in the presence of steam. The effects of doping and steam were more pronounced in the case of synthetic pellets, resulting in an uptake of 23.8 g of CO2/100 g of sorbent after 31 cycles, which represents a conversion of 35.6%. This CO2 capture uptake is very high compared with that of pellets with no CaBr2 addition and no steam present during the reaction cycles, where only 15.0 g of CO2/100 g of sorbent (22.5% conversion) was seen after 10 cycles. These results suggest that the preparation of synthetic sorbents for calcium looping using solutions containing small amount of bromides would be beneficial in practical applications, and steam will either be produced by firing almost any fuel or be found in flue gas suitable for processing by calcium looping.
1. INTRODUCTION Lime-based sorbents have been intensively investigated as sorbents for CO2 capture in looping cycle processes for CO2 capture from large-scale sources to reduce CO2 emissions to the atmosphere and mitigate related climate changes.1,2 Calcium looping (CaL) cycles are based on a heterogeneous gas−solid reaction (carbonation) with the formation of a solid product (CaCO3)
CaCO3 given that the molar volumes of CaCO3 and CaO are 37 and 17 cm3/mol, respectively. However, this is not normally the true limit because the conversion reached after the first carbonation of natural limestone is usually only 70−80% of that limit, and during further cycles, the conversion monotonically decreases, reaching values of 10 g/100 g of sorbent) was captured during this transient period. 7681
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Industrial & Engineering Chemistry Research The results presented in Figure 5 are also interesting in terms of the reaction time, that is, the residence time of the sorbent in the FBC reactors that are anticipated for calcium looping systems.1 Specifically, the typical residence time of sorbent in an FBC carbonator is likely to be only several minutes (potentially shorter). Therefore, only the conversions reached by a sorbent during a short period are important from a practical point of view.43 For example, it can be seen that CO2 capture uptake after 5 min is ∼4 times higher for pellets that are doped with CaBr2 and reacted in the presence of steam than for nondoped sorbent with no steam present. This means that even clearer benefits of doping and the presence of steam are experienced if sorbents are subjected to CO2 capture cycles with shorter carbonation times than employed in this work (20 min). The CO2 capture uptakes achieved by doped pellets in the presence of steam during cycles with 20-, 10-, and 5-min carbonation times are compared in Figure 6. It can be seen that the uptakes in the initial cycles were lower for shorter carbonation times, which is expected, but these uptakes were still high given that the calcination occurred under severe conditions. However, the most important result here is that the differences for these uptakes decreased with increasing numbers of reaction cycles. From the practical point of view, this means that the sorbent activity is more stable over longer series of cycles under realistic conditions and with shorter carbonation times. A likely explanation for this phenomenon is that, with lower conversions for initial cycles, the sorbent is less sintered and therefore more active in subsequent cycles. A similar observation was made by Grasa et al.,44 who found that sorbent partially carbonated during calcination/carbonation cycles demonstrated a higher integral CO2 capture capacity in a longer series of cycles. This is also in agreement with the results of our earlier parametric study on sorbent performance,45 which showed that longer carbonation times cause lower conversions in longer series of calcination/carbonation cycles.
ACKNOWLEDGMENTS
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REFERENCES
This work was partially supported by the European Community’s Seventh Framework Programme (FP7/20072013) under the GA 241302 − CaOling project, together with funding from the interdepartmental Program on Energy Research Development operated by Natural Resources Canada.
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4. CONCLUSIONS The results presented herein, confirm that small amounts of bromides present in a CaO-based sorbent can enhance its activity for CO2 capture in calcium looping cycles in the presence of steam. It is also clear that the diffusion stage of carbonation is enhanced by the presence of bromide ions in the sorbent structure and steam in the gas mixture during calcination/carbonation cycles. Both bromide and steam improve the conversion rate individually, but their synergetic effect results in the highest CO2 capture uptakes. The effect is even more pronounced in the case of synthetic calcium aluminate pellets. Moreover, it is clear that this low halide level also helps alleviate concerns about corrosion or agglomeration in FBC systems. Finally, steam, which is nearly always present in gas mixtures to be decarbonized, should be included in future CO2 tests with synthetic materials because their observed performances appear to be very positively affected by steam.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: b.j.anthony@cranfield.ac.uk. Tel.: 44(01)1234 750111 x 2823. Fax: 44(0)1234 754036. Notes
The authors declare no competing financial interest. 7682
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